The aim of this study was to ascertain the diagnostic yield following a routine genetics clinic referral for the assessment of global developmental delay. Detailed retrospective review of 119 complete consecutive case notes of patients referred to one single clinical geneticist over a 14 month time period was undertaken (n = 119; 54 males, 65 females). The age at initial review ranged from 2 months to 37 years 3 months (mean 8 y 3 mo [SD 7 y 10 mo]). We made a diagnosis in 36/119 (30%); 21/36 were new diagnoses and 15/36 were confirmations of diagnoses. We removed a wrong diagnostic label in 8/119 (7%). In 3/8 we were able to achieve a diagnosis but in 5/8 no alternative diagnosis was reached. We had a better diagnostic rate where the patients were dysmorphic (odds ratio [OR] 1.825; 95% confidence interval [CI] 1.065 to 3.128, p = 0.044). In the majority, the diagnosis was made by clinical examination only. Molecular diagnosis was reached in seven cases. Five cases were confirmed by cytogenetic analysis. Brain magnetic resonance imaging (MRI) revealed a diagnosis in three cases. This study confirms the importance of a clinical genetics assessment in the investigation of global developmental delay.